1. Field of the Invention
The present invention relates to a brush of a rotary electric machine such as a vehicle starter.
2. Description of the Related Art
Because it has been believed that the brush of a dc motor has to have a resistance specific to the output power of the motor, an extensive trial to reduce the resistance of the brush has not been made. For instance, a starter motor for a vehicle usually has a commutator and a pair of or a plurality of brushes made of a mixture of carbon powder and copper powder.
If the content of the copper powder is increased, the resistance of such brushes is reduced. However, the reduction in resistance may cause poorer performance of ac-to-dc conversion by the brushes and the commutator.
JP-A-2002-176750 or U.S. Pat. No. 6,528,923 B2, which is a counterpart of the former, discloses a stacked brush of a low resistance layer and a high resistance layer. The low resistance layer is effective to reduce the resistance of the brush, and the high resistance layer is effective to improve the ac-to-dc conversion or rectification. However, a substantial difference in mechanical characteristics between the two layers may increase as the difference in resistance between the two layers is increased. In this case, the brush may be broken during molding process of the brush or during operation of a motor having the brush, due to high temperature or vibration.